The Theria is a subclass of the class Mammalia, which can be characterized by the possession of well-developed alisphenoids, molar teeth bearing a triangle of cusps (tribosphenoid), cervical ribs fused to vertebrae and a spiral cochlea. The Theria are normal mammals that do not lay eggs but give birth to living young. All extant mammals, except the order Monotremata, are included within the subclass Theria.
The Therian mammals are divided into three infraclasses. The extinct Patriotheria (small, primitive ancestral forms) and the extant Metatheria (the marsupials) with four orders (Marsupicarnivora, carnivorous or omnivorous marsupials; Peramelina, Bandicoots; Paucituberculata, Rat opossums; Diprotodontia, Australian herbivorous marsupials), and Eutheria (the placentals) with 17 orders (Insectivora, divers group of small eutherian mammals; Dermoptera, Colugas or flying lemurs; Chiroptera, Bats; Primates, prosimians, tarsiers, monkeys, apes and man; Edentata, sloths, armadillos and anteaters; Pholidota, Pangolins or scaly anteaters; Mysticeta, baleen whales; Odontoceta, toothed whales; Carnivora, carnivores; Lagomorpha, rabbits, hares and pikas; Rodentia, rodents; Tubulidentata, aardvark; Proboscidea, elephants; Hyracoidea, hyraxes, conies or dassies; Sirenia, manatee, dugong and sea cow; Perissodactyla, odd-toed ungulates: horses, rhinoceroses and tapirs; Artiodactyla, even-toed or cloven-hoofed ungulates)(Subclass Theria). They have their distributions in all faunal regions and all oceans, and occur on the fringes of the Antarctic continent and many oceanic islands.